Case History 2: Tees-Wiske Pipeline
The Tees-Wiske transfer pipeline was designed and installed in 1995-1996 to allow raw water conveyance between the Tyne/Tees and the Ouse River systems. This scheme was instigated to enhance water abstraction to the latter under a drought relief initiative.
The pipeline is a 12km, 1.4m diameter welded construction with an internal lining and high integrity coating. In order to maintain the integrity of the line an impressed current cathodic protection system was installed, with the associated ground bed and transformer-rectifier (TR) located at the northern most end of the line.
Due to topographical restraints, the pipeline course runs close to Railtrack’s East Coast Main Line (ECML) for a significant portion of the route, and crosses a number of minor watercourses and roads.
Following installation of the cathodic protection (CP), it was necessary to carry out interaction testing with the numerous track signaling systems on the ECML to ensure that no adverse stray currents or potential gradients were present.
By switching the Tees-Wiske cathodic protection (operating at full output) it was determined that a low level of interaction was present within the confines of the track circuitry, sufficient to cause some concern. As such further CP commissioning was aborted until the residual voltages within the track circuitry could be reduced.
Following trackside remedial work, the commissioning of the Tees-Wiske CP system was authorised. As part of the commissioning procedure, the natural electrical potential profile of the pipeline was determined by an over the line survey, a baseline survey.
The natural corrosion potential of steel (Fe/Fe2+) in the ground on a copper sulfate electrode (CSE) scale is measured between –500mV to –700mV, with CP subsequently achieved at potentials more negative than –850mV (CSE).
As can be seen from the data (Tees-Wiske pipeline data), normal CP criteria are achieved, albeit erratically, over the majority of the line, without the system having been energised.
However, this survey revealed quite startling effects, with the natural potential of the line varying in the range of –0.8V and –3.0V (as measured on the CSE scale).
The initial consideration for the cause of such potentials was that a stray current was entering the line from the current return circuit on the ECML. Further investigations revealed that whilst DC potentials of up to ~-3V were being experienced, an AC potential profile which peaked at ~-25 to -30V (CSE) was also present on the line.
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